I. Field of the Invention
The present invention pertains to multicomponent, towed-array, marine seismic surveys, and, more particularly, to a technique for collecting and processing data acquired in such a survey.
II. Description of the Related Art
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying acoustic source(s) and acoustic sensors at predetermined locations. The sources impart acoustic waves into the geological formations. The acoustic waves are sometime also referred to as “pressure waves” because of the way they propagate. Features of the geological formation reflect the pressure waves to the sensors. The sensors receive the reflected waves, which are detected, conditioned, and processed to generate seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
Some surveys are known as “marine” surveys because they are conducted in marine environments. Note that marine surveys may be conducted not only in saltwater environments, but also in fresh and brackish waters. Marine surveys come in at least two types. In a first, an array of streamers and sources is towed behind a survey vessel. In a second type, an array of seismic cables, each of which includes multiple sensors, is laid on the ocean floor, or sea bottom, and a source is towed from a survey vessel.
Historically, towed array, marine seismic surveys only employed pressure waves and the receivers detected any passing wavefront. This includes two types of wavefronts. The first are those reflected upward to the receivers from the geological formation. The second are those that are reflected from the surface of the water. The upward traveling wavefronts contain desirable, useful information and the downward traveling wavefronts do not. This sometimes leads to difficulties in processing.
The art has therefore recently begun moving to “multicomponent” surveys in which, for example, not only is the passing of a wavefront detected, but also the direction in which it is propagating. Knowledge of the direction of travel permits determination, for example, of which wavefronts are traveling upward and will yield useful information and which are traveling downwards and will yield undesirable information if confused with upwards traveling waves. Multicomponent towed-array surveys include a plurality of receivers that detect not only the pressure wave, but also the velocity, or time derivatives (i.e., acceleration) thereof, of the passing wavefront. These receivers will hereafter be referred to as “particle motion sensors” because they measure the velocity or acceleration of displaced particles. The pressure sensor is typically a hydrophone, and the particle motion sensors are typically geophones or accelerometers.
The technology is currently immature, however, in many respects. Attention has largely been focused on the equipment and acquisition. However, little attention has been given to the processing. More particularly, the seismic data resulting from this kind of acquisition is useful in conventional types of processing such as group forming, stacking, velocity analysis, moveout correction, etc., by which analysis is typically performed. Nevertheless, this type of acquisition presents unique challenges in pre-processing such data that have not been addressed by the art because the challenges have not previously been presented by conventional acquisition techniques.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.